1. Field of the Invention
The invention relates to image processing apparatus and method for efficiently compressing an image and to a storing medium in which such a method has been stored.
2. Related Background Art
In recent years, computers and networks have remarkably been developed and various information such as character data, image data, audio data, and the like are handled in a computer and in a network.
With respect to the image data and audio data, data of a relatively large data amount is often handled. Therefore, a process to reduce the data amount by compressing the image data and audio data has been performed hitherto.
For example, by compressing the image data, a large quantity of image information can be transmitted through a network at a high speed.
According to the above image compressing technique, a system (generally called a JPEG) for compressing a multivalue still image in the ITU-T Recommendation T.81 is known and a system (generally called a JBIG) for compressing a binary still image in the ITU-T Recommendation T.82 is known.
The JPEG system is suitable for compression of a natural image such as a photograph. The JBIG system is suitable for compression of a binary image such as a black and white character.
According to the conventional JPEG system and JBIG system, all images of multivalue images (photograph or the like) and binary images (character image, diagram image, or the like) cannot be efficiently compressed without deteriorating a picture quality.
To solve the above problem, a method of separating a multivalue image portion and a binary image portion in one picture plane and compressing them by different compressing systems is also known.
The JPEG system is suitable for encoding in a case where natures of images locally differ. By properly switching an encoding system every area in each block, an efficient encoding can be executed while suppressing a deterioration of the picture quality. According to the above system, after multivalue image data was converted into a frequency component every block, by entropy encoding the frequency component, even if the natures of the images locally differ, the encoding according to each block can be executed.
On the other hand, the JBIG system does not execute the encoding on a block unit basis.
However, the system for compressing the multivalue image and binary image as mentioned above does not satisfy all of the requirements of the user yet. Therefore, a new image compressing system is proposed to further improve a picture quality, a processing speed, or an encoding efficiency.